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MG MGB Technical - Bump Steer

I finally got around to sorting out the suspension on my 78B.

To get it back to CB heights I have fitted re-arched springs at the back with the Spax tube dampers Yes, I know people think they are too stiff but will come back to that in a sec.

At the front I have uprated shock valves and have fitted 2 inch lowered and stiffer springs. I have the shortened bump stops in place too. I also fitted an uprated front ARB. I've used blue poly bushes all the way through too.

I know a lot of people think the Spax are too stiff at the back but to me the back seems softer than before. I can't drive the car yet so this is just an impression I get from bouncing the car up and down. My parts guy assures me that the Spax will be good on the rear since I have also (greatly) stiffened the front end. He's fitted them to several cars apparently and they all behave nicely. The front is most definitely stiffer then it was previously.

My real question is about bump steer. Since I still have the RB cross member the lowered springs means the A arms and steering arms now point slightly upwards towards the wheels. If I understand it correctly (and I am not sure I do!) bump steer will be minimised when the arms are parallel to the ground? Am I likely to have bump steer issues with the car as it is now? I've attached a picture so you can see how lowered it is. It's not excessive I think. It is back to CB height and the front wheel is pretty much centered in the wheel arch (which to me looks the most pleasing aesthetically). You can also see the car has a slight lean to the right but it seems a lot of old MGs end up doing this?

Am still a long way off from being able to drive the car so be interesting to see how she works in practice. Haven't ever actually driven a roadster properly either!

Simon




Simon Jansen

I agree with you in that the angles of the steering rods
do not look excessive. I wouldn't worry too much about it.

In the U.S., our cars tend to lean slightly on the left. It's
because of being often driven without a weight-balancing
passenger. Swapping the springs (left side - right side)
helps.
Daniel Wong

Lowering the car may cause some bump steer. I noticed you springs don't look to be seated in the center of the lower spring pan, they are tilted out at the top. The tilt will be worse when the suspension goes into compression on a bump. When you get the car finished it will be lower due to the added weight.

Here is a Pantera article that explains bump steer and how to check and make corrections. You will see the correction may not be easy.
http://www.panteraplace.com/page135.htm

This Longacre Racing article explains bump steer and has a diagram showing how the position of the track rod ends affect bump steer.
http://www.longacreracing.com/articles/art.asp?artid=13

Dave Headley's Grip article is about improving MG suspension. He explains how different changes affect MG handling. Dave is a mechanical with years at GM and lots of racing experience
http://www.fast-mg.com/grip_tech2.htm

Clifton

Clifton Gordon

I should have said "Dave Headley ia a mechanical engineer"

Clifton
Clifton Gordon

With new or worked-on springs front and rear you shouldn't get much if any lean. Only with 'old' springs that have seen lots of heavy driver miles but few passenger miles would you get a 'set' in the springs that could be compensated for by swapping sides. Other causes of leaning are chassis rail damage from accidents, or poor corrosion repairs where things like rear eye centres haven't been adhered to. I've often seen claimed that a *chassis* will take up a set on the drivers side, but find this difficult to believe.

Before I read your comment about the lean I thought the lean I could see was due to camera angles, but if it is leaning as much as it looks something is significantly wrong. You probably don't have the trim-strips on the car, so what is the distance from each hub centre to the crease in the wing above it? I recently fitted new front springs and on buying them I noticed the free heights were different, so asked for a matching pair and got them. But even so whereas before the difference between sides was only 1/8" immediately after fitting it was 1/4" and after shake-down had grown to 1/2". That was over a year ago, haven't measured it lately.

It seems to me you will always get bump-steer to some extent, as the changing angles of the steering arm will cause the wheel to turn in on both a bump and a hollow, if they are dead horizontal statically. However the more they are away from horizontal to begin with (as on this conversion), the more noticeable it will be when they are moved even further away from the horizontal. Having said that the mod is very common and supplied by reputable sources so shouldn't be an issue, certainly not on the road.
Paul Hunt

Clifton, thanks for those links. Interesting stuff! I am not sure why the springs don't look centered properly. They are correctly located. I think it is just because they are so much shorter than the originals. I am hoping the car does lower just a little when everything else is in it. The engine is all in place. Really all that's missing is the screen, seats and all the trim and carpet and so on. The fuel tank is also almost empty so it should squat down some more when I have a full tank of petrol in it.

Paul, the difference in height is about 1/4 to 1/2 an inch side to side. I have the exact measurements from the centers to the bottom of the chrome strips (well, where they will be as you note they aren't there yet!) written down at home. I was going to send them to you once the car is done and everything is settled.

I don't think my car should have a set to it since originally it was LHD. If it did have a lean though I would suspect it would be to the right as mine is as the battery is on that side and the petrol tank is also offset that way too. My car has had a bad shunt in the rear left corner though so that might explain the difference too. There is no noticeable damage to the chassis rails but I wouldn't bet on them being straight!

Simon
Simon Jansen

Tank shouldn't make much difference as it is only biased to one side by about half the width of the silencer. Whilst in theory the battery could account for about 1/2" of deflection of a single rear spring, because they are between the centre of the car and the spring, and forward of the axle, the effect will be significantly reduced. At the end of the day it will be far outweighed (ho ho) by the effects of a single occupant, let alone two.
Paul Hunt

Is it the photo, or have you got lots and lots of negative camber?
Neil
Neil

Neil, I think it's just the photo. Will have a closer look in the weekend. I want to make sure the springs are settled properly and can have a go at measuring the camber somehow if there is an easy way to do that?

Simon
Simon Jansen

Simon, firstly, don't throw your old springs away. Here's my experience. Removed the RBs & fitted "Sebring" front. Replaced worn wishbones with negative camber ones. Fitted S/T uprated 475 LB springs which were about 2" shorter & shorter bump stops. The springs stayed on the car for about 24 HRs & were replaced by the original ones. The bump steer was downright dangerous. The original springs I fitted a couple of years ago are listed on the invoice as "spring coil MGB GT C/Bar red + V8" The distance from the hub centre to the chrome strip is 14" on both sides. The distance from the ground to the top of the rad. grill is 22" & the ground clearance for the susp. X member is 4". I can just get a trolley jack under it. There is still a very slight bump steer, which I can live with & the steering is so much lighter now. Probably because of less tyre contact with the road due to the increased negative camber. This will possibly change as the 195/65 tyres , which are almost new, wear down.
I think your springs are fitted OK. The angles of the X member & wishbones are causing an optical illusion. Good luck, Barie E
Barrie Egerton

Simon
With your heavier springs and uprated swaybar and shockers I doubt very much that your car will suffer from bump steer not from the front anyway. An MGB has a reasonably well worked out suspension for a road car and to get bump steer from the front would have to expierience very long ssuspension movement over bumps which is something I doubt your car will suffer from. I ran 650lb springs and a 3'4 sway bar in a B and had no problems with it at all. A mate has600lb springs and a 7'8 bar and ultra low , no problems with bumpsteer either. Bumpy ride yes bumpsteer no. Just check that the tops of your springs are located up in the crossmember properly there is a little ledge up in there that the spring is supposed to locate around,it is easy to get the spring sitting on the edge of it instead of locating around it--It will make your car sit crooked if not up in there properly,
The best mod. you can do to you rubber bumper car is to the rear. All MGB's but moreso rubber cars suffer from what is called roll steer (or more correctly turn in understeer on a B)which is caused by the way the diff is mounted. This tricks a lot of people into thinking their B understeers which to the driver is what it probably feels like but really you hardly ever see a B understeering off the road or track , they almost always spin out (oversteer)it just feels like it is understeering. It is caused by the diff moving back and forward (steering)in the body as the body rolls over in a corner. To correct this or minimize it in a rubber B it is a simple matter of redrilling the body bracket to relocate the front mounting point of the leaf spring up as high in the bracket as possible.Now your springs will go up and down instead of backwards and forwards, This will also result in lowering the car a bit so the thickness of your lowering blocks might have to be reduced to compensate if it ends up too low. Some spring manufacturers even make a springs with reversed eyes to do this but reverse spring eyes are against anything that I would think to be good. I think that's enough from me for now Cheers Willy
WilliamRevit


I've never noticed any bump steer on my car. There is so much going on in corners anyway. However I've no reason to doubt others were having this problem. My set up was bound to be a little different from theirs. I was interested to read Williams remarks re hanger.
At the back I've; fibre glass springs, antitramp bars,pan hard. Redrilled the rear spring hangers.One inch blocks.
At the front I've got lowered (2") and up rated springs
I was using a 3/4" antisway bar, but had over steer. Quite bad, Frightening in wet weather!
I've now got a 7/8ths (or 22mm) bar and its perfect. There's still a tiny bit of inducible/controllable over steer, which is what you are supposed to have according to the experts.
However I'm using a Toyota gearbox which is allot lighter than standard, so less weight in the front. MInd you, I've less weight in back as well. Springs plus SAAB space saver spare. The 3/4 might be best for your car. It's trail and error for you I'm afraid. I do know where you can get a very cheap 3/4" antisway bar by the way ;)

Do remember to cut about 20mm of your rear bump stops otherwise you get premature contact in corners. This levers weight off the inside wheel, very suddenly, and you get snap over steer which will cause you to fish tail, if you can correct fast enough, or spin out if you don't. Very unpleasant personal experience speaking here. You need at least 2&1/4" upward travel at ride height say the experts. One of you compatriots infact.


Peter

Ah yes, I remembered to cut the rear bump stops but haven't done it yet. Maybe a job for tonight. I will measure for the clearance. I did fit lowered ones at the front o course. I also checked and my springs are OK at the front. I had to take a picture from below using my mobile and an inspection light!
Simon Jansen

This thread was discussed between 05/01/2009 and 11/01/2009

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